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CO2 flow measurement key in CCS schemes

Lynn Hunter, TUV NEL, UK

With energy demand predicted to double over the next two decades and fossil fuels set to supply more than half of the world’s energy needs through to 2030, carbon capture and storage(CCS) is seen as a major contributor to reducing carbon dioxide (CO2) emissions as part of a secure and sustainable global energy supply.

CCS will play a fundamental role in combating global warming and will help those countries that have signed the Kyoto Protocol meet their legally binding greenhouse gas (GHG) reduction targets. Reducing man-made CO2 emissions is a key element in mitigating GHG emissions.

While the Intergovernmental Panel on Climate Change (IPCC) estimates that CO2 emissions must be reduced by 50 per cent to 85 per cent by 2050 compared to 2000 levels, the International Energy Agency (IEA) estimates that early implementation of CCS could reduce global CO2 emissions by 26 per cent by 2050. The UK government estimates that CCS has the potential to reduce CO2 emissions from British fossil fuel power stations alone by up to 90 per cent. At today’s levels this would amount to around 160 million tonnes of captured CO2 every year and approximately one-third of all UK CO2 emissions.

To date, national and international legislation has prohibited the geological storage of CO2, which has been the main barrier in bringing CCS forward. Furthermore, even though the individual components involved in CCS have already been used in various industries, including the oil and gas industry, chemical processing and the food and drinks industry, the full technology chain has never been demonstrated commercially on the scale needed to allow its use in large-scale point emission sources, typical of power stations and other large combustion plants.

As the various governments around the world work together to remove the necessary legislative barriers, immense effort is being deployed in developing and demonstrating the necessary CCS technologies.

It is essential that all CO2 is accurately measured across each stage of the CCS chain, throughout its capture, transportation and storage. This is necessary for environmental purposes ” to detect CO2 leakage and for verification and offsetting under the Eurpean Union Emissions Trading Scheme (EU ETS).

To put the importance of accurate flow measurement into perspective, consider the UK’s largest emitting power station, which emits approximately 22 million tonnes of CO2 per annum. Each percentage point of uncertainty in flow measurement could result in a à‚£6.6 million ($9.8 million) financial exposure in the trading scheme, based on a carbon trading price of à‚£30/tonne as projected between 2013 and 2015.

Despite the anticipated pipeline flow measurement issues and challenges arising from the unique behaviour of CO2 under different property states and process conditions, the majority of CCS research so far has focused primarily on the capture technologies and the geological surveying and monitoring of storage sites.

In order to legalize the storage of CO2 in geological formations and allow emissions abatement from CCS, it has been necessary to both amend and introduce new legislation at various levels. At international level, this has included amendment of the London Convention 1972 and the 1992 OSPAR Convention. The EU and individual member states are currently in the process of establishing the necessary regulatory framework to support the implementation of CCS.


EU Position on CCS


The EU has a legal obligation to meet its Kyoto Protocol commitment to cut GHG emissions in Europe by 20 per cent by 2020 compared to 1990 levels. The EU ETS, together with the proposed CCS Directive and other measures, are expected to keep the EU on track to meet its targets. With regard to CCS, the ambition of the EU is to have 12 fully developed CCS demonstration plants in operation by 2015 in order to allow subsequent commercialization by 2020.

Figure 1: C02 phase diagram Source: TUV NEL

To support this target the EU has set aside €1.05 billion of funding under the European Economic Recovery Plan (EERP) to cover the necessary CCS research and development activities. In the UK, around €180 million of this has been earmarked for the proposed Hatfield integrated gasification combined-cycle (IGCC) power station, owned by Powerfuel Power Limited.

The European Commission published a proposal for a CCS Directive on the 23 January 2008 to establish a legal framework for CCS in the EU. The Directive sets the necessary provisions to take it forward commercially and makes allowances for the storage of CO2 in rock strata beneath EU member state countries and nearby seabeds. Under the Directive two separate permits are planned: an exploration permit, to allow the exploration of suitable geological sites (two year permit with renewable option); and a storage permit, for development and utilization of an approved storage site.

The EU ETS is acknowledged as the key instrument in Europe for controlling and reducing GHG emissions. Now in its second year of Phase II, which runs from 2008 to 2012, the mandatory cap and trade scheme sets limits on the amount of emissions that energy intensive combustion installations are allowed to emit to the atmosphere.

At present the scheme focuses only on CO2 emissions, the largest man-made GHG polluter. Controlled by GHG permits, the EU ETS allows an emissions ceiling to be established whilst providing an incentive for industry to reduce its emissions through the trading of CO2.

CCS will be included in the EU ETS from Phase III onwards which, along with other mechanisms, will provide industry with the much needed financial incentives for introducing CCS. Under the scheme, any captured and permanently stored CO2 will not be considered as emitted. This will introduce substantial savings in both the purchase and trading of CO2. Without CCS, the UK Government estimates that UK industry will spend approximately à‚£8 billion by 2020 on the purchase of EU ETS allowances.

In order to demonstrate the efficient capture and storage of CO2 and allow for offsetting of emissions under the EU ETS, it will be necessary to accurately measure the transfer of CO2 throughout the CCS chain. For this purpose, new monitoring and reporting guidelines have been established under the EU ETS to cover CCS schemes. These guidelines set stringent measurement targets to ensure accurate and credible data are obtained.


UK Position On CCS


The UK is the first country in the world to introduce the necessary legislation to take CCS forward. In addition to the Energy Act 2008, which puts in place the primary legislation and licensing arrangements for the offshore storage of CO2, the Department of Energy and Climate Change (DECC) is running a CCS competition that will fund a large-scale CCS demonstration plant in the UK. The two remaining bidders in this competition, Scottish Power and E.ON, were recently awarded funding for front end enginnering and design studies at longannet and Kingsnorth power plantes respectively. The UK government has also pledged to support up to four full-scale CCS demonstration plants over the forthcoming years.


Flow Measurement Issues and Challenges


There are a large number of potential measurement challenges expected in CCS because of the physical properties of CO2, and the processes and operations involved in CCS schemes. CO2 is unusual because of the relationship and closeness of its triple point and critical point to the temperatures and pressures commonly found in industrial processes. This can be seen in Figure 1.

Compared to other substances that are transported by pipeline (e.g. oil, natural gas and water), the critical point of CO2 lies close to ambient temperature. This means that even small changes in pressure and temperature may lead to rapid and substantial changes in its physical properties (e.g. phase, density, compressibility).

Therefore, not only is there a risk of changing between phases, but also when operating on or close to a phase boundary line, multiphase flow conditions can arise. Phase changes and multiphase flow occurring at measurement points will have a detrimental effect on measurement accuracy, where flow meters are designed to operate in one specific phase only.

In CCS applications, regulating the temperature and pressure will be a difficult undertaking, particularly over long distances. Pipelines will span hundreds of kilometres, and be subjected to various climates and conditions that will naturally affect pressure and temperature, as will CO2 leakage and maintenance operations.

Another major challenge for measurement will be coping with impurities in the CO2 stream. This will be inevitable as the purity levels in the CCS capture processes are unlikely to be greater than 95 per cent. Even trace levels of other contaminants will invalidate the CO2 equations of state and phase diagram, which are based on a pure stream.

Without knowing the exact phase envelope and physical properties of the CO2 stream, it will be extremely difficult to control the CCS processes and undertake accurate flow measurement. It will be necessary to design the flow metering system for the correct physical phase it will be operating in. Accurate density measurements will also be required to allow reporting in mass CO2 units, if the flow meter used measures volumetric flow rate.

There are a number of other factors that may affect measurement. The acoustic attenuation properties of CO2 can affect flow measurement using ultrasonic technology. Large pipeline diameters may also limit some measurement technologies and the corrosiveness of CO2 mixtures may, where applicable, have to be considered during the planning of measurement systems and materials.

There has been a lack of CO2 flow measurement research carried out to support the arrival of CCS. This has mainly been due to the focus on other essential CCS research activities and a possible misperception and lack of awareness of the potential issues and difficulties surrounding the measurement of CO2 in CCS pipelines.

A vast number of potential issues associated with the measurement of CO2 need to be addressed in order to support CCS schemes. Similarly, there is inadequate knowledge of the physical properties and phase envelopes relating to CO2 mixtures in CCS schemes. Such data will be essential for controlling these processes and for planning and designing suitable and accurate measurement systems.

There is a lack of validated data available to support the performance and accuracy of various flow measurement technologies for use in CCS schemes. There are currently inadequate test and calibration facilities worldwide to support the necessary research, development and verification of CCS flow measurement technologies. Finally, there are no flow measurement guidelines and standards at present that are fully transferable to support CCS measurement.


Collaborative approach


It is essential that the development of the necessary measurement technologies is in progress to ensure that timely solutions and options are in place to support the arrival of CCS. It is important that any finalized regulatory monitoring and guidelines for CCS are realistic and achievable, taking into consideration the capabilities and limitations of existing measurement technologies.

There should be greater interaction and collaboration between stakeholders to help support and progress both measurement and CCS. This includes, but is not limited to, power stations owners/operators, regulators, test and research organizations, academia, equipment manufacturers, measurement specialists and CCS support services. In view of this, TUV NEL recently launched a ‘CCS Club’ to address the measurement needs and help promote a collaborative approach between stakeholders.

Under the UK government National Measurement System (NMS), TUV NEL is currently engaged in a CCS measurement research programme addressing a number of the measurement issues and challenges outlined above. As custodian of the UK’s National Flow Measurement Standards and a leading international technology services organization, TUV NEL has a successful track record spanning more than five decades, delivering world class innovative solutions to difficult problems.

The company provides services, solutions and technology to clients across industries, including oil and gas, government, manufacturing, renewable and sustainable energy, on a local and global basis.

Lynn Hunter is a senior consultant at TUV NEL with over 18 years experience in a variety of engineering disciplines. Currently, she leads TUV NEL’s CCS measurement research activities under the UK government’s NMS.



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